a) Field of the Invention
The present invention relates to the treatment of molten metal with a gas in electric arc ovens of the type comprising:
a vat lined with an inner coating of refractory material, said coating being called sole, PA1 lateral walls, PA1 a vault capping the upper opening of the vat, holes being provided in the vault through which electrodes extend, PA1 tapping means (tapping nozzle or eccentric tapping holes). PA1 the energy supplied by the electric arcs is absorbed mainly in the zone of the electrodes and in the upper part of the metallic bath, which brings about a non-uniform distribution of the temperature of the metallic bath. This non-uniform distribution leads to an increase of the electric energy required for melting the steel located in the colder zones. PA1 in, an arc oven, the bath of liquid steel is not very high and covers a large surface area which contributes to the fact that liquid steel has a heterogenous chemical composition. PA1 blowing of oxygen during periods of melting or refining, brings about an overoxidation of the iron at the surface of the metallic bath, PA1 the chemical reactions between metal and slag take place mainly at the surface of the metallic bath and are not influenced by the weak movements of liquid steel.
It is customary to use a gas to ensure the stirring of molten metal.
b) Description of Prior Art
Currently, in laddles for molten steel, devices for injecting inert gases through the bottom are used. In this case, the utilisation of an inert gas gives a better homogenization of liquid steel, with respect to chemical composition and temperature. Similarly, in converters where oxygen is blown from the top, and an inert gas is blown from the bottom, and gas stirring is specifically used to homogenize the molten metal and to accelerate chemical reactions between metal and slag.
The melting of scrap iron with an electric arc oven has many disadvantages, which, inter alia, are the following:
It is usually recognized that these disadvantages can be minimized by stirring steel from the bottom of the arc oven.
Many devices for stirring molten steel are known. According to a known process, liquid steel in a laddle is set in motion by introducing an inert gas through the bottom by means of a porous refractory material. This process utilizes a refractory brick or a porous plug which has the property of being permeable to a gas under pressure but is impermeable towards molten metal. The porous plug is part of the refractory coating of the laddle. It is mounted so as to be covered with liquid metal when the laddle is full. Stirring is obtained by injecting a gas through the porous plug or through the refractory brick.
Up to now the application of this technology to an arc oven has not been industrially developed because the lifespan (at most 40 cycles in continuous operation) of the porous plugs or refractory bricks is insufficient in view of the restraints inherent to the exploitation of an arc oven which prevent the device for injecting gas to be replaced without loss of productivity.
According to another known process, which uses a converter where oxygen is blown from the top, the stirring of the molten metal with an inert gas is made possible by means of a device made of a body of non-porous material having localized discontinuities spreading along its entire height in the direction of the gaseous blowing and permitting the gas to flow. Up to now these devices for stirring molten metal were not significantly used in arc ovens because of the specific requirements of the latter. Indeed, the thermic restraints of the refractory material of the sole of an arc oven increase with the thinness of the sole as compared to the bottom of a converter. Moreover, when pouring of the liquid steel is carried out through an eccentric tapping hole, there are normally some remains of liquid metal or bath bottom in the oven, which cover the opening of the injection devices and prevent them from being inspected. These disadvantages are still increased in an arc oven as a result of the importance of the movements of gas and liquid metal which, with the fall of scrap iron when loading, produces important mechanical restraints. In addition, there is a chemical erosion of the refractory material of the sole of the arc oven resulting from chemical reactions between refractory material, liquid metal and slag. For all these reasons, the stirring techniques with an inert gas used in a converter could not be directly applied to an arc oven.
There is also a procedure for stirring molten metal in an arc oven by using an electromagnetic effect. This process is implemented by means of a device mounted underneath the vat of the arc oven. It has the disadvantages that the stirring power has some limitation and of requiring costly investments.
Another process, described in British Patent No. 2,192,446, enables to stir the liquid steel at the bottom of the arc oven through an injection device comprising a brick of refractory material through which small tubes extend along the entire length of the brick in the direction of blowing. In this device, the small metallic tubes are in direct contact with the liquid metal during a melting cycle in an arc oven, which prevents from electrically insulating the metallic enclosure of the oven from the mass of molten metal without adding a system of electrical insulation which is specific to the injection device. Moreover, when the injection device is damaged, it must be replaced but this can only be done when the electric oven is stopped, which results in a loss of productivity.
Another process, described in European Patent No. 0240998, enables the stirring of liquid steel at the bottom of the arc oven by means of tuyeres. In this process, it is not possible to stop using or to start to use the gas again during a cycle.
It is an object of the present invention to improve the technique of injecting gas at the bottom of an arc oven while minimizing the risks of break of the sole and enabling to rapidly repair the devices for injecting gas when they are worn out, without having to cool the sole of the oven and without any loss of productivity. Another object of the invention is to permit an electrical insulation between the molten metal and the metallic frame of the arc oven which is simple and natural.